Phase modulation



Aug. 1, 1944. M. s. CROSBY I PHASE MODULATION .or iginal Filed July 26, 1940' CARR/ER WA VE SOURCE I MODULHTED XmI xuj

INVENTQR Y B I m Y W R Y 4. A w "MT U Y 8 Patented Aug. 1, 1944 PHASE MODULATION Murray G. Crosby, Riverside, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Original application July 26, 1940, Serial No. 347,587, now Patent No. 2,316,155, dated April 13, 1943. Divided and this application January 26, 1943, Serial No. 473,595

7 Claims. (01. 179-1715) This application, which is a division of my United States application No. 347,587, filed July 26, 1940, now Patent #2,316,155, dated April 13, 1943, concerns a new and improved phase modulation means of the type wherein an element of a phase shifter is varied in value to produce a variable phase shift and consequently produce phase modulation. A phase shifter utilizing inductance and resistance is employed so that the resistance may take the form of a tube plate resistance which may be modulated in accordance with the signal voltage. Phase modulation has been accomplished heretofore by controlling the value of a circuit element at signal frequency to thereby control the phase of current therein. In my system I use a mid-tapped inductance and a resistance and vary the resistance. My circuit will produce a greater phase shift for a given amount of resistance variation than certain known systems of this nature. My system also makes a greater range of total phase variation possible. One of its most outstanding advantages is the fact that the phase shift is accomplished without amplitude change. that the phase modulation will not be accompanied by concomitant amplitude modulation which must be limited off by means of a limiter following the modulator stage.

In describing my invention, reference will be made to the attached drawing wherein;

Fig. 1 shows an embodiment of a phase modulator arranged in accordance with my invention; while Fig. 2 illustrates the essential elements of the phase shifter and is used with the vector diagrams of Fig. 3 to illustrate the operation of my system.

In the phase modulator ofthis disclosure a phase shifter such as shown in Fig. 2 is utilized. This phase shifter shifts the phase of voltage El by an amount depending upon the magnitude of the reactances of LI and L2 and resistance R. The vector diagram of this type of phase shifter is shown in Fig. 3. The voltage to be phase-shifted is applied as El. El causes a current to flow through Ll and R so that a reactance drop XLII will appear across points O-A of Fig. 2 and a resistance drop RI will appear across terminals +0 of Fig. 2. A reactance drop XmI will appear across terminals 0-3 of Fig. 2 by virtue of the mutual inductive coupling which exists between Ll and L2. Voltage El is the resultant of the reactance and resistance drops XLlI and RI.

The output voltage of the phase shifter E2 is the resultant of the mutual reactance drop Km]: and

the resistive drop RI. It can be seen that El is different in phase by an amount 0 depending upon the relative Values of the resistive voltage dropRI and the reactive voltage drops XL-II and XmI. Consequently, by varying the value of R, the phase may be varied between zero' and almost 180 degrees.

This means From the vector diagram of Fig. 3, it can be seen that as Rv is varied between zero and a value equal to the reactive impedance, the phase shift, 0, will be varied between zero and degrees and the amplitude of the output voltage, E2, will not vary since E2 is always equal to El so that there is no variation of the attenuation with modulation. Consequently, phase modulation is possible without the introduction of concomitant amplitude modulation.

The phase shifter of Fig. 2 is adapted to use as a phase modulator by utilizing a resistance at R which can be controlled at a modulation potential rate. If the phase of the current is to be keyed or modulated, the. value of the resistance R may be varied by keying signals which shunt a portion of R or all of R by a second resistanceto change its value in accordance with the signal. In a preferred embodiment the resistance R is comprised of the resistance of a vacuum tube illustrated in Fig. 1. Hence, by applying modulating potentials to the vacuum tube, the phase of the voltage transmitted through the phase shifter is varied in accordance with the modulating potentials since, as is well known, the plate resistance of a vacuum tube may be varied by varying an electrode potential. Circuits having these features are shown in Figs. 1 and 2.

In the circuit of Fig. 1 constant carrier energy is supplied from source III by transformer l2 to the grid l4 of amplifier 20. The output circuit connected with the anode 22 of the amplifier tube 20 comprises a tuned circuit 4, wherein the amplified wave appears, and from which it is fed to the phase shifter consisting of inductance 6 and the resistance between the plate 50 and cathode 52 of diode tube 54. The phase-controlled output of the phase shifter is fed to additional stages such as amplifiers and frequency multipliers and transmission means not shown.

Modulating potentials are applied to jack 3| and then through transformer 32 to the anode of tube 54. Bias is furnished for the diode 54 by means of battery 51, and a blocking and coupling condenser 58 is in series with the diode impedance.

In the circuit of Fig. 1 the resistance between the anode 50 and cathode 52 of a diode tube 54 3|, ,transiormer 32 and the radio-frequency choke RFC. Condenser 58 is a blocking condenser to keep the plate voltage from amplifier tube 20 from being applied to diode 54. Battery 51 biases the diode to. a mid-position on its resistance-voltage characteristic so that the diode resistance may be modulated at a higher or lower value than this mid-position value.

As described in my United States application Serial No. 36,473 filed August 16, 1935, now United States Patent No. 2,104,318 issued January 4, 1938, several of these phase modulators may be cascaded. in order to produce an increased or moreiaithful degree of phase modulation.

What isclaimed is:

1. Means ior'shifting the phase of alternating current comprising-a. reactance energized by wave energy, a diode tube, means connecting an electrode of the diode to a point on said reactance, means for impressing voltages the phase or which is to be shifted between a point on said reactance and the other electrode of said diode, and means for deriving voltages the phase of which may be varied by'varying the impedance of said diode from another point on said reactance .and said other electrode of said. diode.

2. Means for shifting the phase of alternating current comprising a 'reactive circuit, a diode having one electrode connected to .a point on said reactive circuit, means for impressing voltages the phase of which is to be shifted between one terminal of said reactive circuit and the other electrode of said diode, connections for applying a direct current bias potential across the electrodes of said diode, and means for deriving voltages, the phase of which may be varied by varying the value of said bias potential from the other terminal of said reactive circuit and the other terminal of said resistance.

3. Means for shifting the phase of alternating current comprising an inductance, a diode tube having an electrode connected to a point on said inductance, connections for impressing voltages the phase of which is to be shifted between said inductance and the other electrode of said diode-a source of potential connected between the electrodes of said diode, and means for deriving voltages, the phase of which may be varied by varying the value of said potential from said inductance.

4. In a system of the class described, a source of wave energy the phase of which is to be shifted, areactance, a diode having an electrode connected to a point on said reactance, means for impressing current from said source on said reactance and on one electrode of said diode, means for deriving phase shifted current from said reactance and the other electrode of said diode, and means for applying a variable potential between theelectrodes of said diode to shift the phase 01' the said derived current.

5. In a phase modulation system a source of wave energy the phase of which is to be modulated in accordance with control potentials, a reactance,-a diode tube having one electrode connected to a point on said inductance, means for impressing current from said source on said reactance and the other electrode of said diode, means for deriving phase shifted current from said reactance and said other electrode of said diode, and means for applying a variable potential to the electrodes of said diode.

6. In \a phase modulation system, a source of current the phase of which is to be modulated, a source of modulating potentials, an inductance, an electron discharge device having two electrodes one of which is connected to a. point on said inductance, means for impressing current from said source of current the phase of which is to be modulated on said inductance and another electrode of said tube, means for modulating the impedance of said tube in accordance with potentials from said source of modulating potentials, and means for deriving modulated current from said inductance.

7. In a phase modulationsystem, a source of current the phase of which is to be modulated, a source of modulating potentials, an inductance, an electron discharge device having an anode and a cathode, means connecting one of said electrodes to a point on said inductance, means for impressing current from said source of current the phase of which is to be modulated on said inductance and the other electrode of said tube, means for modulating the impedance oi said tube in accordance with potentials from.

said source of modulating potentials, and means for deriving modulated current from said inductance.

MURRAY G. CROSBY. 

